Study On Synthesis Optimization Method And Miniaturization Of Symmetric Coupler

The symmetrical directional coupler has been widely used in microwave and millimeter wave circuit design. With the development of communication technology, both military and civilian frequency bands have a tendency to expand to a wider band. Thus, there are higher requirements for the band and the performance of the symmetrical directional coupler. However, it is difficult for the single parallel line coupler to realize the broadband and high performance. Therefore, we need to design a step symmetric coupler to broaden the frequency band and improve the performance.In the design of the step symmetric directional coupler, it is very difficult to obtain a good result in a short time, so it is necessary to propose a fast method for the optimization of the symmetric coupler. Based on the results of previous research, this paper presents a novel optimization method for the design of the symmetric coupler. In this method, Legendre polynomials are chosen to be the basis functions to expand the ripple polynomials, of which the expand coefficients are obtained by the quasi-Newton method and conjugate gradient method. As shown in the numerical examples, the convergence rate obtained by the proposed method is far faster than the one obtained by the traditional power series expansion method. Moreover, the proposed method can optimize the performance of the extremely high-order symmetric couplers.In this paper, to verify the integrated optimization method, a broadband coupler is designed in the frequency range from 0.5 to 3GHz. Synthesis optimization method can obtain perfect results in the design of the ideal coupled line circuit model directly, but this method is not considered the connection between coupling lines, so the model is necessary to be further optimized in the HFSS. Besides, there are discontinuous between the intersegmental couplings in the actual circuit model which is eliminated by the introduction of the capacitor loaded stub. The simulation and experiment results show that the improvement is very effective.The miniaturization is one of the trends of the current coupler development. At present, the bend line method is employed in the design of the reverse directional coupler. This method will not worsen the general performance of the coupler despite that it will deteriorate the isolation of the coupler. Moreover, this method also has many limitations. For example, it is generally suitable for the frequency band below 1GHz and its application in the step coupler will raise the difficulty of wiring.In this paper, a novel method for the design of a dual band coupler is proposed, which is proved to be feasible. However, due to the introduction of the PIN diode, the performance of the coupler is deteriorated. Although the reconstruction method will affect the performance of the coupler, but it can realize a coupler with discrete bands and reduce the size of the coupler 67% or more, which the traditional design method cannot be achieved.